Synthesis And Crystal Structures Of All Thiophene-based [7]helicenes With Isomeric Location Of Sulfur Atoms

Helicene has been widely applied in the fields of asymmetric catalysis,circular polarization luminescence(CPL)and chiral recognition due to its unique helical structure and chirality.As one of the important parts of heterohelicenes,thiophene-based helicene has made considerable progress in recent years.In this thesis,based on the theory of structure-activity relationship,three thiophene-based[7]helicenes with isomeric positions of sulfur atoms in the two terminal thiophene rings of molecule are designed and synthesized by cross-coupling synthetic strategy with three dithienothiophenes(DTTs)as the building blocks.The crystal structures and absorption behaviors were studied to reveal the effects of the location of sulfur atoms in the two terminal thiophene rings on the molecular packing pattern and absorption spectrum behavior.The specific research content includes the following two aspects:1.Synthesis of thiophene-based [7]helicenes and DitrithienothiepaneIn this thesis,three thiophene-based [7]helicenes(UH-1,UH-2 and UH-3)with isomeric sulfur atoms on the terminal thiophene rings of molecules were synthesized for the first time through key reactions such as cross-coupling of three dithienothiophene(bb-DTT,bt-DTT,bs-DTT)and cyclization of bi-DTT with(Ph SO2)2S.Two new heterocyclic isomers,ditrithienothiepanes(DTTTP-1 and DTTTP-2)were obtained simultaneously.1).Synthesis of thiophene-based [7]helicene(UH-1,UH-2 and UH-3)Compound endo-exo-UH-1 was synthesized,with dithieno[2,3-b:3',2'-d]thiophenes(bb-DTT)and dithieno[2,3-b:2',3'-d]thiophenes(bt-DTT)as the building blocks.First,top-endo-bidithienothiophene(9)was generated from bb-DTT and bt-DTT by Suzuki coupling,then LDA was introduced to remove its protons and following cyclization with(Ph SO2)2S was occurred to generate endo-exo-UH-1 in a total yieldof 16%.Through the similar method of making endo-exo-UH-1,using bt-DTT and bs-DTT as building blocks,the key intermediate top-endo-bidithienothiophene(12)was obtained.After the deprotonation of 12 in presence of 1 equivalent of LDA,quenching with TMSCl,and the top-endo-bidithienothiophene(13)bearing TMS group was selectively generated in a yield of 93%.After deprotonation of 13 in presence of 3equivalents of LDA and following cyclization with(Ph SO2)2S,top-endo-UH-2 was obtained in a total yield of 6%.By the same way,intermediate top-exo-bidithienothiophene(15)was obtained with bb-DTT and bs-DTT as building blocks.Similarly,top-exo-bidithienothiophene(16)bearing TMS group was produced with high selectivity in 95% yield.After deprotonation of 16 in presence of LDA and following cyclization with(Ph SO2)2S,the compound top-exo-UH-3 was obtained in a total yield of 24%.The compounds UH-1,UH-2,and UH-3 are isomers with the same molecular skeleton and different location of sulfur atom in the two terminal thiophene rings of molecules.2).Synthesis of ditrithienothiepane(DTTTP-1 and DTTTP-2)Compound top-endo-DTTTP-1 was synthesized from the precursor 13 of UH-2 with a yield of40% by removing protons of 13 in presence of 2 equivalents LDA and following cyclization with(Ph SO2)2S.When using 3 equivalents of LDA,UH-2 and DTTTP-1 can be obtained in one-pot simultaneously with yields of 11% and 49%,respectively.Similarly,the precursor 16 of UH-3 was used as the raw material,after deprotonation of 16 in presence of 2 equivalents of LDA,and cyclization with(Ph SO2)2S,and UH-3 and DTTTP-2 were obtained in one-pot in yields of 26% for both.When 3equivalents LDA were used,both UH-3 and DTTTP-2 were obtained in yields of 46% and 18%,respectively.UH and DTTTP are isomers with different molecular skeletons.The experimental results show that molecules 13 and 16 have three active ?-protons,which are selectively deprotonated under LDA,and the formed tricarbanions show the selectivity of cyclization with(Ph SO2)2S,and generate the selectiveyield distribution of UH and DTTTP.2.Crystal structures and absorption behaviors of thiophene-based [7]helicenes and ditrithienot hiepaneThe molecular structures UH-1,UH-2,UH-3 and DTTTP-1 were confirmed by X-ray single crystal diffraction analysis.The single crystal structure shows that UH-1 belongs to the triclinic system,P1 space group;UH-2 and UH-3 belong to monoclinic system,C2 space group,and monoclinic system,C2/c space group,respectively,Their molecular skeletons exhibit helical structures.UH-1,UH-2 and UH-3show different locations of sulfur atoms in the two terminal thiophene rings of molecules,resulting in the different intramolecular repulsion between two terminal thiophene rings.The dihedral angle between the two terminal thiophene rings of UH-1 is 38.8o,which is smaller than 41.2o of UH-2 and 39.9° of UH-3.UH-1 has a helix climb of 2.09 ?,and turns in-plane by 237o,which are smaller than the helix climbs(2.36?,2.17 ?),and the turns in-plane(261o,273o)of UH-2 and UH-3,respectively.In the crystal stacking,there are multiple intermolecular interactions such as S???S and S???C between the molecules UH-1.Although UH-2 and UH-3 have similar stacking methods,which are different from UH-1.Their intermolecular interactions are different,such as S???S,S???H,C???H and ????? interactions are observed for UH-2,and S???S,S???C,C???H and S???H are found for UH-3.DTTTP-1 belongs to the monoclinic system,C21 space group,which has an approximately planar structure.The crystal packing diagram of DTTTP-1reveals the intermolecular multiple interactions such as S???S,S???C,S???H and C???H.These crystal data show that location of sulfur atom in the two terminal thiophene rings of molecules for thiophene-based[7]helicenes can indeed affect their geometry of the molecular skeletons and their intermolecular interactions,resulting different stacking modes.The UV-vis absorption spectra properties of UH-1,UH-2,UH-3,DTTTP-1 and DTTTP-2 werestudied.Compared with the reported maximum absorption peak(267 nm)of thiophene-based [7]helicene,whose all sulfur atoms are on the outer edge of molecule.Bearing one sulfur atom on the terminal thiophene ring isomerized to the inside position of the molecule,UH-1 shows an absorption peak red shifted to 273 nm,and a significant expansion of the absorption peak.UH-3,bearing one sulfur atom isomerized to the top of the molecule,exhibit two shoulder peaks at 280 and 330 nm.The two terminal sulfur atoms of UH-2 are isomerized to the inside and top positions,respectively,showing the strongest molecular conjugation effect.The maximum absorption peak shifted to 289 nm with a strong absorption shoulder peak appeared at 330 nm.These absorption behaviors mean that the isomerization of the sulfur atom position is the key role to turn molecular conjugation degree.The sequence of conjugation effect is shown as UH-2 > UH-3 > UH-1.The main absorption of DTTTP-1 are peaked at 286,327 and 390 nm,while DTTTP-2 not only has an absorption peak at 327 nm,but also a shoulder peaked at 386 nm.The difference in molecular structure leads to the difference in the absorption spectra between DTTTP-1 and DTTTP-2,that is,the sulfur atoms of the dithienothiophene building block of DTTTP-2 are on the outer edge,and the conjugation effect of the molecule should be poor.Compared with the helical structure of thiophene-based[7]helicene,the molecular skeleton of DTTTP shows more planar structure,and its conjugation degree is much greater than that of thiophene-based [7]helicene,revealing significant red shifts of its absorption spectrum.